Current limiting device



May 19, 1970 TAKEO lNwE ETAL 3,513,426

' I CURRENT, LIMTING DEVICE Filed Feb. '25, .11969 United States Patent O 3,513,426 CURRENT LIMITING DEVICE Takeo Inowe, Takashi Shirasawa, Takashi Nislnraku, and Toshio Miyamoto, Amagasaki, Japan, assignors to Mitsubishi Denki Kabushiki Kaisha, Tokyo, Japan Filed Feb. 25, 1969, Ser. No. 802,188 Claims priority, application Japan, Feb. 29, 1968, 43/ 13,032 Int. Cl. H01h 61/00, 83/00,' Hlc 7/00 U.S. Cl. 337-219 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates in general to a current limiting device, and more particularly a current limiting fuse of self-restoring type including sodium, potassium or a solid solution thereof as a current limiting material.

The self-restoring type of current limiting devices called permanent fuse capable of repeatedly performing the current limiting Operations includes, as a current limiting material, a metal in the form of a liquid or a solid solution readily liquefiable at room temperature and operates to respond to a fiow of shortcircuiting current through the current limiting material to evaporize the material through the generation of Joule's heat therein due to the fiow of shortcircuiting current whereby a plasma with very high vapor pressure is formed. Then the evaporated material and the plasma present to that fiow of current a specific resistance much higher than the shortcircuiting impedance of the associated circuit thereby to limit the shortcircuiting current below a predetermned magnitude. After the completion of the current limiting operation, the material is rapidly cooled or agitated by a turbulence to be liquidized or solidified whereupon the material is self-restored to its original, good electrically conductive state.

It is commonly practced to use any one of sodium .(Na), potassium' (K), an alloy thereof (NaK) etc. In any event, the particular current limiting material must greatly increase in vapor pressure upon responding to a fiow of shortcircuiting current therethrough to be evaporized to perform a current limiting operation. Also an enclosure having disposed therein such a current limiting material undergoes a very high 'pressure upon evaporating the material and therefore it must particularly have a mechanical strength suflicient to withstand such pressure.

SUMMARY OF THE INVENTION Accordingly it is an object of the invention to provide a current limiting device including a new and improved enclosure surrounding a body of current lim-iting material and having a mechanical strength sufi'icient to withstand a very high pressure developed in operation while readily providing the required electrical characteristics.

It is another object of the invention to provide a current limiting device including a new and improved composition of current limiting material.

With these objects in view, the invention resides in a current limiting device comprising a body of current limiting material responsive to a fiow of current therethrough above a predetermned magnitude to be evaporated thereby to perform a current limiting operation and restored to the original state after the completion of the current limiting operation, and a pressure proof enclosure for surrounding the body of current limiting material to be subject to a pressure upon evaporating the material, characterized in that the pressure proof enclosure includes a pair of concave and convex metallic members in the form of hollow cylinders, one of the cylindrical members having one end portion of reduced diameter loosely inserted into the other cylindrical member to form a gap therebetween, and a solid material fills the gap and the interior of the cylindrical members except for the body of current limiting material to unit the cylindrical members into a unitary structure, the last-mentioned solid material being selected from the group consisting of electrically insulating materials and electrically resisting materials.

Preferably the one end portion of the one cylindrical member may gradually increase in outside diameter to- Ward the extremity thereof While the adjacent end portion of the other cylindrical member is substantially complementary in shape to the one end portion of the one cylindrical member.

Advantageously, the solid material may include an electrically resisting material and a pair of electrodes are electrically connected to the remote ends of the united cylindrical members to electrically connect the electrically resisting material in parallel circuit relationship with the body of current limiting material through the pair of electrodes connected to the body of current limiting material.

The solid material may conveniently include mica having a particle size less than meshes, a low melting point glass enriched a borate and including a metallic oxide selected from the group consisting of barium o xide and strontium oxide, and a graphite having a partlcle size of from 60 to 140 meshes, the graphite being included in an amount of approximately 20% on the basis of the volume of the solid material for small-sized current limiting devices.

Alternatively the solid material may include a thermoset resinous material consisting essentially of an aromatic compound having high resistances to both sodium and potassium, and fibrous graphite whose content is approximately 40% on the basis of the solid material for large-sized current limiting devices.

BRIEF DESCRIPTION OF rH-IE DRAWING The invention Will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a fragmental longitudinal sectional view of a current limiting device constructed in accordance with the principles of the invention;

FIG. 2 is a longitudinal sectional view of the device being processed in accordance with the principles of the invention;

FIG. 3 is a fragmental longitudinal sectional view of a modification of the invention; and

FIG. 4 is a longitudinal sectional view of another modification of the invention.

Throughout the figures like reference numerals designate the corresponding components.

3 DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing and in particular to FIG. 1, there is illustrated a current limiting device constructed in accordance of the principles of the invention. The device comprises a pair of reinforcing tubular members and 12 substantially equal in outside and inside diameters to each other and disposed in end-to-end relationship. One of the tubular members, for example, the member .10 has one end portion 14 decreased in outside diameter and loosely inserted into the adjacent end portion 1.6 of the other tubular member 12 substantially complementary in shape of the one end portion 14 of the tubular member 10 to form a gap 18 therebetween. The tubular members may be formed of any suitable metallc material having a mechanical strength sufficient only to withstand a molding pressure at a predetermined molding temperature upon molding the device as will be described hereinafter. Such a material is not critical in coeflicient of thermal expansion and required only to be high in mechanical strength at room temperature. Suitable examples of the material involve iron, stainless steels, etc.

As shown in FIG. 1, a corrosion resisting, electrically insulating material 20 as will be fully described hereinafter fills both the gap 18 between and an internal cylindrical space defined by the internal peripheral surfaces of the tubular members 10 and 12 except for a central bore 22 which will be subsequently filled with a current limiting material such as previously described.

A moulding device as shown in FIG. 2 may be conveniently used to prepare the arrangement as above described in conjunction with FIG. 1. The moulding device comprises a moulding box 24 and a three part mould 26 snugly fitted into the box 24. With the lower end :as viewed in FIG. 2 of the mould 26 closed by a support block 28, the tubular members 10 and 12 are filled one above the other into the mould 26 until the adjacent ends of both the members are somewhat separated away from each as shown at solid line in FIG. 2. Then the electrically insulating material 20 in the form of a powder fills the remaining portion of the mould 26 as shown at dotted lines alternating solid lines in FIG. 2. Thereafter the upper end of the mould is closed by a push block 30.

The assembly thus produced is heated at a predetermined temperature at which the insulating material is moulded. Then the assembly is placed in any suitable press. While the predetermined temperature is maintained, the assembly is pressed under a proper pressure until the tubular members 10 is brought into its predetermined position as shown at dotted line in FIG. 2 as well as the insulating material has been moulded.

Thereafter the pressed assembly along with the moulding device is allowed to be cooled while it is maintained under the pressure. Then the moulding device is dismantled and the resulting moulding is removed from the mould. The moulding is machined to be formed with a central bore 22 extending therethrough. Thus a pressure proof sleeve has been completed. It has been found that the insulating material 22 solidified and bonded to the adjacent end portions and the internal peripheral surfaces of both the tubular members 10 and 12 is high in pressure resisting property and ensures that the tubular metallc members 10 and 12 are electrically insulated from each other while both the members are united into a unitary structure.

A current limiting element (not shown in FIG. 1) complementary in shape to the bore 22 is threaded through the bore 26 to complete a current limiting device. The current limiting element is composed of any one of sodium, potassium and solid solution thereof.

FIG. 3 shows a modification of the invention Wherein the tubular member 10 has one end portion 1-4 of reduced diameter gradually ncreased in outside diameter toward the extremity thereof, that is to say, in the form of an inverted wedge. The end portion 14 of the tubular member 10 is loosely inserted into the adjacent end portion .16 of the other tubular member 12 substantially 'complementary in shape to the end portion 14. In other respects the arrangement is identical to that shown in FIG. 1. It can well withstand an axial tensile stress.

FIG. 4 shows another modification of the invention wherein the metallc tubular members 10 and 12 also serve as electrodes for a fuse device. The arrangement is identical to that illustrated in FIG. 3 except for a pair of electrode terminals 32 and 34 being rigidly secured to the tubular members 10 and 12 at the free ends respectively as by bolting or brazing. If desired, either of the electrode terminals 32 and 34 may have incorporated thereinto butfer means for decreasing a pressure within the tubular members upon a current limiting material involved being evaporated and a valve through which the current limiting material is charged within the members although they are not illustrated.

According to the teachings of the invention, the corrosion resisting, electrically insulating material as previously described may be preferably produced starting with either an inorganic mixture in the form of a powder consistng of natural or synthetic mica, a low meltingpoint glass and graphite or an organic mixture in the form of a powder consisting of a thermosettable resinous material and graphite.

If it is desired to produce a corrosion resisting, electrically insulating material of the inorganic type, then a mixture of powdered mica, low melting-point glass and graphite is charged into any suitable moulding machine and heated at 500 C. in any suitable electric furnace for 30 minutes after which it is moulded under a pressure of 1,500 kg/cm.2 while it is maintained at the specified temperature. The moulded material is then finely pulverized ready for use with the invention.

The type and composition of the starting material greatly aifects the corrosion resistance of the resulting material which will be subsequently described.

It has been previously practiced to use a powder of mica having a relatively large particle size, for example, of 60 to meshes for the purpose of providing mica moulding including a low melting point glass as a binder and high in mechanical strength and also to use in many cases the lead borate system glass excellent in mechanical characteristics. If such a mica moulding as produced is used as a pressure proof inner wall (or cylinder) it will have been corroded to a considerable extent for various reasons. Among them the most serious reason is the presence of lead oxde in a glass involved. It is wellknown that any glass containing lead oxde is much attacked with alkali metals such as the elements sodium, potassium, etc. For example, this attacking of glass is remarkably observed with vitreous enclosures for sodium vapor lamps.

Therefore an increase in alkali resistance of a particular glass requires to include an oxide(s) of alkali metal(s) or alkaline earth metal(s) in the glass as the essential basic component. I-Iowever it is to be noted that among alkaline earth metals, only barium, strontium are available because the remaining of alkaline earth metals such as calcium, magnesium are high in melting point and apt to be devitrified. Also as to the acidic component of glass it is required to essentially used boric acid While a content of silicic acid is maintained as low as possible. In addition to meeting these requirements for the glass composition, the glass is required to have a viscosity permitting it to fiow at 500 C. while at the same time preventing the devitrification thereof during its cooling from 500'I C. down to room temperature. A particular glass meeting the last-mentioned requirement may react by itself upon moisture or carbon dioxide gas in the air to suddenly decrease in surface resistance. It has' been determined that this phenomenon occurs substantially negligibly with mouldings of such glass because it can contact the air with an extremely decreased contact area. It has been also found that the phenomenon is greatly governed by the particle size of powdered mica involved and that the smaller the particle size of powdered mica the less the occurrence of the phenomenon will be. Thus it is preferable to use a powder of mica having a particle size equal to or less than 140 meshes. Such powdered mica may have a content of from 70 to 30% by volume with a content of glass ranging from 20 to 50% by volume.

It has been found that by properly selecting a glass composition, a particle size of powdered mica and/or a proportion of glass to mica, a mixture consisting only of the glass and mica is possible to provide a pressure proof sleeve capable of being satisfactorily put of practical use. However such a sleeve is not yet satisfactory in corrosion resistance. The addition of powdered graphite to such a mixture plays a useful role in readily and greatly increasing the corrosion resistance of the resulting material. It has been found that the use of powdered graphite having a particle size of from 60 to 140 meshes and a content of approximately 20% on the basis of the total volume of the mixture is very advantageous in that a corrosion loss decreases by a factor of two without varying the moulding ability, 'mechanical strength, etc. The powdered graphite may have its content up to 40% by volume with the satisfactory results. The resulting material decreases in electric resistance as the graphite increases in content. Since the addition of graphite in a content of 30% by volume provides an electric volume resistivity in the order of 109 S2/ cm., a material including graphite in a content up to 30% by volume may be regarded as being an electrically insulating material. The addition of graphite in a content of from 30 to 40% by volume causes a further decrease in electric resistance and provides the so-called electrically resisting material. It has been found that if a content of graphite exceeds 40% by volume the resulting material decrease in mechanical strengths too greatly to be of practical use.

The organic type of corrosion resisting, electrically insulating material may be prepared from thermosettable resinous materials and graphite. A mixture consisting of a selected one of thermosettable resinous material in the form of a powder and a powder of graphite is charged into any suitable moulding machine and heated at 500 C. in any suitable electric furnace for 30 minutes after which it is moulded under a pressure of 510 kg./cm.2 while it is maintained at that temperature. The moulded material is then finely pulverized ready for use with the invention.

Like the inorganic type of corrosion resisting electrically insulating materials, the organic type materials have the corrosion resistances very alfected by the type and composition of their starting materials. First it is essential that the thermosettable resinous materials themselves be excellently resistant to both sodium and potassium. As well-known, the thermosettable resinous materials excellent in resistance to both sodium and potassium are ones including essentially an aromatic compound. For example it has been found that the use of polymethylene diphenyloxide gave the very satisfactory result.

Graphite is mixed with a thermosettable resinous material involved for the same reasons as above described in terms of the inorganic type materials. It has been found however that the use of fibrous graphite results in great improvements in moulding ability and mechanical strength. A proportion of graphite to the particular thermosettable resinous material has the close bearing upon the moulding ability and corrosion resistance of the re-4 sulting material. It has been determined that a higher proportion of graphite to a thermosettable resinous material aids in increasing the corrosion resistance but adversely affects the moulding ability of the resulting material. In other words, the content of graphite governs the uniformity of density distribution of the moulding. Thus it has been found that as in the inorganic type materials as above described should have a graphite content u-p to .'approximately 40% on the basis of the volume of a starting mixture although the content of graphite is not allowed to be extremely high.

The results of experiments indicated that any corrosion resisting, electrically insulating material, whether it is of the inorganic or organic type, produced in the manner as above described is far improved in not only the static load characteristics such as the bending strength but also the impact load characteristics as compared with porcelain materials previously used. This is because the effect of graphite used as a filling material is to increase the elastic coefficient of the resulting material. Therefore the corrosion resisting, electrically insulating materials according to the teachings of the invention can hardly damage due to thermal shock. In addition, as an enclosure composed of a pair of tubular metallic members united together as previously described is adapted to accommodate a vapor pressure developed therein due to the evaporation of a current limiting material involved, any damage to the insulating material can be quite prevented by properly determining the dimensions of the tubular metallic members.

Further the present insulating materials are far Superior in corrosion resistance to the conventional porcelain material including a large amount of silicate poor in alkali resistance for the reasons that the present materials include graphite completely resisting to both sodium and potassium.

The inorganic type materials can be advantageously used in conjunction with small-sized devices to decrease the manufacturing costs whereas the use of the organic type materials with large-sized devices is advantageous in that the problem of dielectric strength can readily be solved. Thus whether the inorganic type or the organic type is used is determined for the particular application.

While the invention has been described in terms of a corrosion resisting, electrically insulating material for electrically insulating a pair of tubular metallic members involved from each other, it is to be understood that the insulating material may be replaced by an electrically resisting material having a suitable magnitude of resistance. More specifically an inorganic or organic type material such as previously described can have a suitable magnitude of resistance by mixing either the particular powdered mica and glass or the particular thermosettable resinous material with graphite suitably selected in both amount and particle size as previously described. That is graphite can be used in a content of from 30' to 40% by volume. Then the electrically resisting material thus prepared is then disposed between and within the particular pair of tubular metallic members in the manner as previously described to electrically connect an electrical resistance between the metallic member due to the electrically resisting material in parallel circuit relationship with a body of current limiting material involved through the associated pair of electrodes such as shown at 32 and 34 in FIG. 4. The measure is effective for suppressing any extraordinary voltage developed between the electrodes upon limiting a shortcircuiting current.

What we claim is:

1. A current limiting device comprising a body of current limiting material responsive to a flow of current through above a predetermined magnitude to be evapo- -rated thereby to perform a current limiting operation and restored to its original state after the completion of the current limiting operation, a pressure proof enclosure for surrounding said body of current limiting material to be subject to a pressure upon evaporating said current limiting material, said enclosure including a pair of concave and convex metallic members in the form of hollow cylinders, one of said cylindrical members having one end portion of reduced diameter loosely inserted into the adjacent end portion of the other cylindrical member to form a gap therebetween, and a solid material filling said gap and the interior of said cylindrical members except for said body of current limiting material to unite said cylindrical members into a untary structure, the lastmentioned solid material being selected from the group consisting of electrically insulating material and electrically resisting materials.

2. A current limiting device as claimed in claim 1, wherein said one. end portion of the one cylindrical member gradually increases in outside diameter toward the extremity thereof While said adjacent end portion of the other cylindrical member is substantially complementary in Shape to said end portion of the one cylindrical member.

3. A current limiting device as claimed in claim 1, wherein said solid material is an electrically ressting material and wherein a pair of metallic electrodes are electrically connected to the remote ends of said cylindrical members to electrically connect said electrically resistng material in parallel circuit relationship with said body of current limiting material through said pair of electrodes connected to said body of current limiting material.

4. A current limiting device as claimed n claim 1,

Wherein said solid material includes mica having a particle size equal to 'at most 140 meshes, a low melting point glass enriched in borate and including an oxide selected from the group consisting of barium oxide and strontium oxide, and graphite having a 'particle size of from to meshes, said graphite being included in an amount of approximately 20% on the basis of the volume of said solid material.

5. A current limiting device as claimed in claim 1, Wherein said solid material includes a thermoset resinous material consisting essentially of an aromatic compound having high resistances to -both sodium and potassium, and fibrous graphite whose content is approximately 40% on the basis of the volume of said solid material.

References Cited UNITED STATES PATENTS 2,543,177 2/1951 Korsgren 337-290 X 2,728,836 12/1955 Boisblanc et al 338-30 3,1l7,203 1/1964 Hurtle 337-119 3,454,833 7/1969 Hurtle 337-119 HIRAM B. GILSON, Primary Examiner U.S. Cl. X.R. 337-290; 338-30 

